In earlier work it has been shown that a promising approach for transporting real-time video traffic in packet switching networks is to use feedback from the network to modulate the source rate of a video encoder. However, a major problem exhibited by these earlier schemes is unfairness in the service provided to users--where the grade of service is defined in terms of the quality of the video signal delivered to the user.
Two distinct approaches are currently utilized for supporting real-time video over packet-switched networks. One approach is for the video encoder to produce a constant bit rate data stream with the network offering guarantees of constant bandwidth to each video connection. An alternative approach is to allow the encoder to generate a variable bit rate data stream, thereby allowing for more efficient use of network resources due to statistical multiplexing gains. Simple variable bit rate encoders operate in an open loop mode in which the value of the quantizer step size is kept constant, thereby assuring a constant quality video signal to end users, as long as there is no data loss. However, network overloads can cause data loss due to buffer overflows or excessive delays with a consequent degradation in image quality.
The degradation in image quality during overloads can be controlled by modulating the source bit rate of a video encoder based on the state of the network. This results in more efficient use of the network bandwidth and a more graceful degradation in image quality during periods of congestion. However, a drawback is that during periods of overload, sources with more complex image sequences see a greater reduction in image quality. A divergence in image quality of more than 5 dB between sources has been observed, resulting in clearly visible differences in the picture quality. This unfairness occurs because while more complex image sequences require a greater amount of bandwidth to obtain the same level of quality, the feedback control schemes attempt to achieve parity in the bandwidth used by each source.
There is a need for a new feedback control scheme that provides significantly greater fairness while still allowing for efficient usage of network bandwidth and graceful degradation in image quality under overloads.